Plastec: Lights…camera…cure; LSR cures via UV, allowing PP overmolding

Anaheim—If it was injection molded liquid silicone rubber (LSR), it was cured via heat, until now. "LSR first launched in the 1970s, and it's always been thermally cured," explained Sharon Shatto, Americas Marketing Manager, Healthcare for Momentive Performance Materials during Plastec West 2013.

Momentive used an Elmet tool with a transparent acrylic cavity and UV LED lights to cure its new UV curable LSR on an Engel e-motion machine. Silicone typically cures at temperatures above 120°C, limiting overmolded thermoplastics to materials like nylon, but the bottle stopper molded on the  e-motion 310/110 LIM injection molding machine featured a polypropylene insert.

LSR PP bottle stopperShatto said that Momentive introduced a UV curable LSR to the extrusion market two years ago, with curing there made easier by placing UV lighting at the die exit. Getting UV lighting into an injection molding tool required some recent advances in lighting technology as well as a machined PMMA cavity insert.

"If someone had come to us 6-8 months ago and asked if we could overmold LSR onto a PP, we would have said, 'We couldn't do that, we'd melt the plastic,'" Shatto said. In the intervening months, LED advances, including smaller lights with more specific wavelengths, have given Momentive the right tool to do the job.

At the show, the PP part that serves as the top of the stopper is molded in the lower cavity of the tool. A robot with two end-of-arm-tools (EOAT) enters the cavity, pulls the PP part, and places it into the top half of the mold, where the LSR is overmolded. The second EOAT removes finished stoppers from the top cavity. While the LSR is overmolded in the top half, a new PP insert is injected in the bottom half of the tool.

The tool and acrylic cavity insert are made by Elmet, and at the show, they ran a  53-55 second cycle, slowing the process down to allow attendees to see inside the mold. Finished parts exit the press and are cool to the touch, with the silicone cured in only 10 to 11 seconds and no added heat required. Normally LSR set ups have a 1:1 mix ratio of material to catalyst, but in this setup the ratio is 98:2.

The PMMA insert is built into the mold cavity itself, allowing UV light to pass through and cure the LSR. Tool temperatures are set in the range of 30-40°C, and cavity pressure remains low since there isn't the normal expansion that occurs with heated LSR. In fact, the expected cavity pressure was less than 50 bar.

Potential new applications utilizing the technology could include IV components, respiratory masks, or products where silicones could be overmolded onto sensitive electronics. Another potential application would allow incorporation of a variety of active ingredients into the silicone that normally would not survive LSR curing temperatures.

Colored parts are possible, as long as they're not opaque, since light must travel through the LSR for curing. Right now, Shatto said Momentive is offering 60 and 30 durometer grades, with more in the works.

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